1. Field of the Invention
This invention relates to the encoding and decoding of multiplexed analog components (MAC) video signals.
2. Description of the Prior Art
For many years, color television (TV) signals have been transmitted using the PAL, NTSC and SECAM color systems. Due to the huge amounts of monochrome equipment in existence when these three systems were initially contemplated, economic and bandwidth considerations imposed a large restraint on their designs. Specifically, the systems had to be compatible with existing monochrome systems in that they had to transmit their signals over existing channels and in that their signals had to be such that they could be received (in monochrome) on existing monochrome receivers as well as being received (in color) on color receivers. As a result, the systems were so designed that the chrominance (color) information was transmitted within the same frequency band as the luminance information, the chrominance and luminance information or signals being combined to form a so-called composite signal. This is possible by virtue of the fact that, by modulating the chrominance information onto a color sub-carrier of a precisely controlled frequency, it is possible to interleave the frequency spectra of the chrominance and luminance signals so that they suffer minimal interference (cross-talk) with one another. In practice, a certain amount of cross-talk (cross-color and cross-luminance) does in fact occur, at least in some circumstances.
In more recent times, with the advent of direct broadcast by satellite (DBS) systems, which are not subject to the constraint of having to be sent by existing (terrestrial) channels, the MAC system was designed. There are several variants of the MAC system, including normal definition MAC, widescreen MAC, high definition MAC, B-MAC, C-MAC, D-MAC, D2-MAC and so on. All of the variants are characterized by the fact that, instead of being sent in composite form, the chrominance and luminance signals are sent sequentially, that is on a time division multiplex basis, so that they cannot interfere with each other. Thus, cross-color and cross-luminance cannot occur.
A full description of the MAC system can be obtained from various published documents, including the following specification published by the European Broadcasting Union (EBU): "Specification of the systems of the MAC/packet family-Tech 3258-E", EBU Technical Centre, Brussels, October 1986. For present purposes, the relevant features of the MAC system are as follows. Each line (that is, each line scanning interval) of the transmitted signal includes time-compressed chrominance information and time-compressed luminance information, the chrominance and luminance information being sent one after the other. In similar manner to existing systems, the chrominance information comprises two color difference signals. However, the two color difference signals are not both sent during each line. Rather, to reduce transmission bandwidth, the respective two color difference signals are sent on a line sequential basis, that is during alternate lines. Thus, odd-numbered lines of a frame will contain one of the color difference signals and even-numbered lines of the frame will contain the other of the color difference signals. However, since the number of lines per frame is odd, for example 625 for normal definition MAC, to simplify the design of equipment used in the system the sequence of the color difference signals is reset between frames so that each one of the color difference signals is always sent during lines of the same number in successive frames. That is, the (n)th line of each successive frame will always include one of the color difference signals and the (n+1)th line of each successive frame will always include the other of the color difference signals.
Prior to transmission, in an encoder for encoding signals into the MAC format, the color difference signals are vertically sub-sampled to achieve the desired line sequential color difference information. That is to say, alternate vertically adjacent samples of each of the color difference signals are discarded to leave, for each line, samples for one only of the color difference signals. Sub-sampling resulting in the elimination of every other sample will, of course, result in halving of the vertical Nyquist frequency. In the absence of any corrective measure, this would lead to vertical chrominance aliasing. So, the color difference signals also have to be vertically prefiltered so that their spectra are restricted to a band substantially below the halved vertical Nyquist frequency. The filtering can be effected in an analog or digital finite impulse response (FIR) filter of conventional design, using appropriate weighting coefficients to achieve the desired bandwidth reduction. Thus, in summary, the color difference signals are subjected to a process known as "decimation" (passed through a "decimation filter"), in which the color difference signals are vertically filtered and vertically sub-sampled.
At the receivers, in a decoder for decoding the received signals from the MAC format into a form suitable for display, the recovered sub-sampled color difference signals are interpolated to estimate the color difference information that was discarded, prior to transmission, in the sub-sampling operation. That is to say, an interpolation filter estimates, for each line, that one of the two color difference signals that was not transmitted for that line by taking averages between that one of the two color difference signals transmitted during other lines, for example the two lines immediately above and below it. The transmitted luminance signal, the transmitted color difference signals and the estimated color difference signals can then be converted into a conventional format (such as RGB) for display.
The above-described operation of prefiltering the color difference signals before transmission will, of course, involve some loss of frequency response and therefore, in principle, some degradation in picture quality. In practice, it has been found that the degradation is so slight as to be virtually undetectable to the eye, even under stringent test conditions. This is because the eye's response to color information is poor so that, for example, information about picture edges is given largely by luminance information only. A similar (negligible) degree of picture degradation results from the operation of filtering carried out at the receiver by the interpolating filter. Thus, it was not anticipated that the need to filter the color difference signals prior to transmission, and the need to filter them on reception (in the interpolating filter), would give rise to any significant picture degradation problem.
At present, with the MAC system being fairly new, most studio equipment for use in MAC studios is of a conventional type, the signal being put into MAC format immediately before transmission. However, as the use of MAC systems becomes more extensive, it is probable that studio equipment specially designed to encode or convert signals into MAC format and/or to handle signals in MAC format will become generally available. (In particular, to take one example, it is probable that video tape recorders (VTRs) that can convert a signal into MAC format and store a signal in MAC format will be widely used.) This will involve signals being passed through MAC encoders and/or MAC decoders on multiple occasions. For example, MAC format VTRs may be used to make multi-generation copies of program material. Thus, the above-mentioned absence of any anticipation that the need to filter the signals would give rise to any significant picture degradation problem, which in retrospect seems to have involved an assumption (probably an unconscious assumption) that the filtering operation would be performed only once, will give rise to problems.
In the foregoing regard, tests conducted by the inventor have produced the following results. As indicated above, picture degradation in the case of one pass through the decimation filter and the interpolating filter is neglible. However, the frequency response is cumulative in the event of plural passes and reduced vertical chrominance resolution and color smearing of the picture are in fact observable in the event of plural passes. Picture degradation is clearly observable in the event of two passes and, after three or more passes, the picture looks "soft". In the event of more than three passes, the picture degradation is a serious problem.
Thus, the signal degradation experienced in the event of multiple passes through the decimation filter and/or the interpolating filter represents a very serious problem which poses a severe obstacle to the successful development of MAC systems.
An object of the invention is to solve the signal degradation problem explained above.